a high level of back-drivability and a good isotropy (manipulability index = 0.23±0.02; force/torque ratio = 2.21±0.19N/Nm)with a large available force level at the handle (continuous force > 50N; peak force > 200N) allowing forexperiencing a wide range of haptic stimuli. The controller consisted

of three nested loops with a 16 kHzsampling rate (current loop) and 1 kHz rate (impedance control loop), plus a 100 Hz virtual reality loop.Software environmentwas based on RT-Lab®

and Simulink®.

The end-effector moved along an arc starting from point P1, passing through P3 and ending at P2(Fig.1

C) and then returning to P1. The motion law is:

)1())/2cos(sin())/2cos(cos(TtRYYTtRXXCEECEE

,whereXEE

andYEE

are the moving end effectorCartesiancoordinates,XC

andYCare the coordinates ofthe circlecenter

generating the haptic contour,R

is the radius of the circle,

is the central angle of the arc,T

is the duration of the entire movement (T=3 s) andΔt

is theactual movement duration.

In thepassive condition

the robot delivered a two-component force field to the hand: an attractiveforce that smoothly moved the hand along the virtual surface and a viscous force field for the stabilization ofthesubject’sarmwhile interacting with the device(equation 2):

dtdXBBXXXXXXKFHHTHTHT00

(2)

, whereXH

andXT

are, respectively, the actual coordinates of hand and target position at a preciseinstant,B

is the derivative gain (10Ns/m) andK

is the stiffness (60 N/m).

This controlschemeallowed forthe generation ofastereotyped biological

speed profile, characterized by a symmetric

shape

with

a singlebell-shapedvelocitypeak

and an acceleration and deceleration phase, whichmimicked the profiles seenduring active motion in humans.The complete exploration of each curvature lasted 3 s and consisted offorward

motionalong the curved surfacefollowed bya

motionbackward

along the same path.

In theactive

condition

the range of curvatures and the dimensions of the hand paths were identical tothe passive condition. Participants actively moved their hand along a virtual contour

at a speedsimilar to thepassivecondition.The appropriate boundary forces